Optical fibers are generally constructed with a glass cylindrical core encased within one or more layers of cladding, and light pulses are transmitted through the core of the optical fiber. The light rays or modes of a pulse, typically from a laser diode or light emitting diode, follow different paths within the optical fiber core as they reflect back and forth along the boundary of the core and cladding. Since the pulse length has a tendency to elongate during travel along the core and thereby restrain the bandwidth, optical fibers have been manufactured with their core having an index of refraction profile that varies radially from the axis of the core to the periphery to facilitate telecommunication applications. The refractive index distribution within the optical fiber core should be designed so as to cause all light rays of a pulse to travel along the optical fiber at the same axial velocity regardless of variations in the length of the path traversed. In practice, optical fiber manufacturing processes introduce some deviation from optimum refractive index distribution of the optical fiber core. Therefore, the variation from an optimal refractive index distribution must be consistently monitored to ensure that the variation remains within certain predetermined acceptable limits.
A number of methods have been developed and are known for analyzing the refractive index profile of optical fibers. A good review of the various optical fiber and preform index-profiling methods are disclosed in an article by W. J. Stewart titled "Optical Fiber and Preform Profiling Technology", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 10 (October, 1982). Perhaps the most widely accepted method in use today is the refracted near-field method described in the previously noted article. With this particular method a lens having a numerical aperture substantially larger than that of the fiber focuses a beam of light on a flat endface of a fiber and scans the focused spot across the fiber diameter. Part of the light is guided down the fiber while the rest is refracted through an end portion of the fiber and radiates as a hollow cone outside of the fiber. A shield or disc is placed in the radiated cone to prevent the leaky modes in addition to the purely reflected modes from reaching a photodetector which is positioned beyond the disc. A detailed review of this method is set forth in an article titled "Practical Application of the Refracted Near-Field Technique for the Measurment of Optical Fiber Refractive Index Profiles" by K. I. White which was published in the March, 1979 issue of Optical and Quantum Electronics.
However, the refracted near-field technique suffers from the practical problem of requiring highly sophisticated optical equipment and thereby renders obtaining refractive index profiles a complex and expensive task. The improved method of the present invention is directed to obviating the requirement for sophisticated optical equipment or complex procedures in order to obtain accurate refractive index profiles of optical fibers.